It is well known to package materials in laminar or sheet-like plastic films or multi-layer plastic film laminates. Where the materials are sensitive to oxygen and/or water vapour (e.g., materials such as certain foodstuffs, beverages, chemicals, pharmaceuticals, seeds, electrical components, etc.), a plastic packaging material is chosen which will provide a barrier against ingress of oxygen and/or water vapour, or other gases. It is also well known to package certain foodstuffs in an atmosphere of gas contained within a plastic material chosen for its low permeability to that gas (controlled atmosphere packaging or modified atmosphere packaging sometimes known as MAP).
It is also known to apply a gas-barrier layer, for instance a thin layer of a metal such as aluminum, in order to improve the barrier of the film to oxygen, and other gases and/or to water vapour. Metallized films can be further laminated to a heat sealable film such as a polyolefin film (e.g., polyethylene or polypropylene) or they can be laminated to a polyester film, to produce a material suitable for packaging oxygen—or moisture-sensitive products. However, the gas and moisture barrier properties are not significantly improved by this lamination. It is also known to apply gas barrier layers, such as layers of ethylene vinyl alcohol copolymers or layers of polyvinylidene chloride.
For some applications it is desirable to decorate the package with, for example, print, and for reasons of security and integrity the printed material is frequently sandwiched between layers of the laminate. This can be achieved by printing the film, and metallizing over the print before further conversion. These activities are typically performed by film converters who process films for supply to the ultimate users. This procedure has the undesirable effect that the printed film becomes more permeable to oxygen and water vapour than the unprinted metallized film.
Metallized films are known to have low permeability to moisture but they have an undesirably high permeability to oxygen. Examples of such films include metallized polyolefin films, such as metallized mono-oriented or bi-axially oriented polypropylene film (hereafter OPP), metallized polyethylene film (hereafter PE), or metallized oriented polyester film (hereafter PET). Single webs of these films typically provide a moisture barrier of about 1 g/m2/24 hours (at 38° C., 90% Relative Humidity). Laminates of such metallized polyolefin films to unmetallized films, suitable for packaging of moisture-sensitive materials, are described in GB patent 1,566,925. However, gas and moisture barriers are not significantly improved by this lamination when compared with those of the single web metallized film. The clear web itself or the adhesive used for the lamination can, however, be chosen to impart good barrier properties.
In some applications it is desirable to decorate the package made from laminates of metallized and unmetallized films with, for example, print, and the print is often sandwiched between the layers of the laminate. This is normally achieved by printing the clear web and laminating it to the metallized polyolefin web. Gas and moisture barrier properties are not significantly improved over that of the single web metallized film unless the clear web itself or the ink or the adhesive has good barrier properties. The decorated package could also be produced by printing the clear polyolefin web and metallizing over the print, and then laminating to another clear polyolefin web, but this is not used commercially, the former process described above being preferred. By laminating a metallized polyolefin film to a further web of a metallized polyolefin film, further reductions in permeability can be achieved and a material with both good moisture barrier and good oxygen barrier is produced. European patent 154 428 A describes such laminates. The terms “coating” and “precoating” as used herein include the presence of laminar sheet elements in the film structure as well as coatings applied as liquids, melts or solid granular additives. For example, an intermediate or external layer may be considered as a coating or precoating depending upon its use. Similarly the terms “coating” and “precoating” may also include tie layers or skin layers depending upon their intended use.
Polypropylene films with a coating on one or both sides and metallized on one of the coated surfaces are also known from, for example, U.S. Pat. No. 6,013,353, these films are commercially available. When the coating is of a thermoplastic polymer resin that has no particular barrier properties, such as an acrylic resin, oxygen permeability of the unmetallized coated film is undesirably high (typically 500-1000 cc/m2/24 hours at 23° C. and 0% Relative Humidity) and oxygen permeability after metallization is also correspondingly high (greater than 10 cc/m2/24 hours at 23° C. and 0% Relative Humidity). When the coating is of a thermoplastic polymer resin with good barrier properties, such as a polyvinylidene chloride resin, oxygen permeability of the coated film is significantly reduced (typically 25 cc/m2/24 hours at 23° C. and 0% Relative Humidity) and oxygen permeability after metallization is correspondingly low (typically less than 5 cc/m2/24 hours at 23° C. and 0% Relative Humidity). Such films are widely used for packaging either in single web form or laminated to another unmetallized web. For applications where it is desirable to decorate the package with, for example, print, and to sandwich said print within a laminate, the unmetallized web is normally printed and then laminated to the metallized coated polypropylene web.
Current metallized films suffer from the disadvantages that scratches and folds in the metal, typically aluminum, layer can significantly decrease the water vapour barrier and gas barrier properties of the metallized films. Furthermore, the aluminum layer is difficult to print, and needs to be primed before printing, and, in addition, the aluminum layer is not sealable. A further disadvantage is that the unprotected aluminum layer is very sensitive to scratching and oxidation which is detrimental to the optical quality and barrier properties of the film and for most applications the aluminum layer has to be protected by the converters with a varnish. U.S. Pat. No. 6,013,353 describes a process in which a coating is applied on top of the metal layer of a metallized film. The films of U.S. Pat. No. 6,013,353 are however described as having only moderate oxygen barrier properties.
It is known that thin aluminum layers such as those present in metallized films oxidize to some degree upon exposure to the atmosphere to produce aluminum oxide. It is also known that the presence of aluminum oxide in the aluminum layer is undesirable in that it is hygroscopic and reduces the barrier properties of the layer, thereby reducing barrier properties to both oxygen and water vapour. Accordingly in a process such as that described in U.S. Pat. No. 6,013,353 there can be significant oxidation of the aluminum layer.